1. Field of the Invention
This invention relates to power supply circuits, and particularly uninterrupted power supply (UPS) circuits adapted to have paralleled DC-to-AC inverters across a load.
2. Description of the Prior Art
Paralleling of UPS systems, in particular DC-to-AC inverters, is known in the prior art. Such paralleling is used to provide redundancy in situations calling for stringent maintenance of power to critical loads such as computer systems, aircraft, and the like. Thus, a redundant system consists of at least two independent supplies which are connected in parallel, each of which supplies can carry the entire load. If one UPS fails for any reason, the other or others maintain system operation. In other applications, paralleling is required to provide the capacity to service a given load. For example, two or more UPS units of a given or different ratings may be needed to service a given load. Alternately, in a situation where one UPS may be able to handle a load when operating at maximum output, two or more units may be used in parallel to achieve the desired capacity, providing both redundancy and greater reliability due to operating each supply below its rated capacity.
Typically, paralleling of the output DC-to-AC inverters has been accomplished by controlling the amplitude and phase of the reference voltage to the inverter using a demodulator and modulator scheme. This scheme is costly and does not help in transient sharing; only the fundamental component of the current is shared. Further, this arrangement is generally not viable for paralleling inverters of different power ratings without substantial circuit changes.
An example of an attempt to balance load supplied by plural UPS units is disclosed in U.S. Pat. No. 4,114,048. There, the circuit is designed to balance load current through the respective inverters, but not load provided at the inverter outputs. Also, this design makes no attempt to balance harmonics.
Present day UPS applications have made it more critical that paralleled UPS units be able to provide balanced sharing of harmonics. Many UPS loads, such as computer loads, include significant transient conditions, resulting in harmonics. If such harmonics are not shared properly, but are distributed in an unbalanced way, the lifetime of a UPS unit can be significantly reduced. In some instances, unbalanced sharing of harmonics can even cause relatively quick failure. There is, thus, a need in the art for a UPS design enabling harmonic paralleling of the DC-to-AC inverter outputs of UPSs, enabling each UPS to share harmonic-rich loads such as presented by computers. What is needed is a UPS design that provides for sharing of subharmonic and higher harmonic currents, thereby enabling paralleling for supplying a nonlinear load. At the same time, there is a need for a simple and efficient manner of paralleling inverter outputs of UPS units so that each unit shares the load current as a ratio of its power rating to the power rating of the other parallel inverters.